Add jerryscript-compiler.h public header to cover compiler incompatibilities (#2313)

In general, public headers should not have compiler-specific
constructs but both the core and the port headers have attributes,
which are non-standard. It's better to factor out such constructs
to a common place (a new header) and hide them behind macros, which
can then be defined on a per-compiler basis.

This patch moves the existing definitions of function attributes and
likely/unlikely builtins to the new header. At the same time, it
unifies the names of these attribute defines and where they are
used. Moreover, it touches on jerry-main and removes the uses of
`__attribute__((unused))` entirely and replaces them with the
elsewhere used `(void) ...` pattern.

JerryScript-DCO-1.0-Signed-off-by: Akos Kiss akiss@inf.u-szeged.hu

jerry-core/ecma/operations/ecma-regexp-object.c

@@ -302,7 +302,7 @@ ecma_op_create_regexp_object (ecma_string_t *pattern_p, /**< input pattern */
  *
  * @return ecma_char_t canonicalized character
  */
-inline ecma_char_t __attr_always_inline___
+inline ecma_char_t JERRY_ATTR_ALWAYS_INLINE
 re_canonicalize (ecma_char_t ch, /**< character */
                  bool is_ignorecase) /**< IgnoreCase flag */
 {